Preheat your oven to 375°F (190°C) and lightly grease a small baking dish.
Slice the tops off the bell peppers, remove the seeds and membranes, and rinse them thoroughly.
In a large skillet over medium heat, add the olive oil and sauté the diced yellow onion until translucent, about 5 minutes.
Add the minced garlic and ground beef to the skillet, breaking the meat apart with a spoon until it is fully browned and cooked through.
Stir in the cooked brown rice, tomato sauce, dried oregano, sea salt, and black pepper, simmering for 2 minutes to meld the flavors.
Stand the bell peppers upright in the prepared baking dish and spoon the beef and rice mixture into each pepper until tightly packed.
Sprinkle the tops with parmesan cheese, cover the dish with foil, and bake for 30 minutes.
Remove the foil and bake for an additional 10-15 minutes until the peppers are tender and the cheese is golden.